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US$80150 per caput for simplified sewage (1988 dollars). Actual costs for selected projects in Brazil appear in Table 11.4. |
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11.6
Operational Experience |
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Since their first implementation in the Brazilian states of São Paulo and Paraná, simplified sewerage has been subsequently applied in Bolivia (Cochabamba and Oruro), Colombia (Bogota and Cartegena) and Cuba (Matanzas). Although no specific data on operational problems are readily available, it is nevertheless known that no significant problems have been reported. In the city of São Paulo, it has been estimated that there are 75 obstructions per 1000 km of sewers each month. This infrequent occurrence of obstruction gives further support to the policy of minimizing the number of manholes. Engineers in SABESP consider that it would be economical to install only a few manholes initially, with the intention of building additional ones if the need arises (i.e. at points of frequent obstructions). Similarly no problems related to excess hydrogen sulphide generation have been reported from field surveys. |
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The objective of this chapter was to present information on simplified sewerage which provides a new cost-saving approach to the design of sewer systems, based on Brazilian experience. It is based mainly on rational changes in long-standing traditional sewer design standards. The present conventional engineering practice in sewer design was introduced more than a century ago and has undergone relatively few significant changes since. Engineers in Brazil, who more than a decade ago took a serious look at the rationale for the various design criteria, have found ample room for change and simplification without jeopardizing the operational integrity and safety of the system. |
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Engineering design is not conceived exclusively on the basis of rigid and exact scientific facts but is often based heavily on empirical data supplemented with probabilistic and risk criteria. The factors of safety which have been embedded in many design criteria (design flow, minimum diameter, depth of sewers, etc.) |
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